Without knowing specifically what frequency you're trying to tame, the angles of incidence involved, what your room is like, etc., it's hard to say. In general, you want something which resists movement of the wave but still allows it to pass. 'Fluffy' stuff will certainly let it pass but unless you have 6" or more of it, it does very little to impede the wave-flow.
If your room is overly bright, it's likely that frequency related issues are not your only problem. I'd recommend at a minimum, hitting your first reflection points on the side walls with some 2" OC 703 spaced off the wall 1-2". This will give you good control, better imaging, and help tame things a little bit from an implulse response perspective.
Then to tame the brightness a bit more and also help control front wall reflections, do the same thing on the front wall at a minimum, behind and between the speakers. Doing the whole wall is not overkill.
